The present invention relates to a method for producing half-tone reproductions (preferably printing blocks or color separations) of an original on a recording medium by means of a recording element using rotated screens having any screen angle and any screen line spacing and being formed of periodically repeated, adjacent screen grid elements comprising screen dots varying in size with the tone values of the original. The original is optoelectronically scanned line-wise and pixel-by-pixel, dot-by-dot, or point-by-point for producing an image signal and by line-wise recording by means of a recording element displaced over and with respect to a recording medium, the latter having associated with it an orthogonal co-ordinate system subdivided into areal elements and aligned in the line direction. The locus co-ordinates of the areal elements traversed momentarily by the recording element are determined continuously and a recording signal is generated for the recording element by current comparison of the image signal with a screen threshold signal, the recording signal controlling the recording of the individul screen dots as a configuration of areal elements in the co-ordinate system. The invention also relates to a system for carrying out the method.
The inventive method is applicable for example in the case of a color scanner for producing corrected color separations. In the case of a color scanner of this nature, which is known per se, a colored original is scanned point by point and line by line by means of an optoelectronic scanning element and three primary color signals are concomitantly obtained which are converted in a color computer into the color-corrected color separation signals for recording the colour components "magenta", "cyan" and "yellow".
Recording elements in the form of light sources modulated in brightness by the color separation signals, peform the point-by-point and line-by-line exposure of the color components on a photosensitive recording medium. The color separations may be produced as half-tone color separations for further processing in engraving machines or else as screen color printing blocks if they are to be applied as forms for color offset printing.
The printing in superimposition of the differently inked screen printing blocks of a colour separation for multicolor reproduction is then performed in a printing machine.
A moire pattern is generated since it is impossible in practice to print the screen dots of the individual component colors precisely on each other. A moire pattern of this nature is disturbing particularly upon inspecting the finished printed picture.
The obtrusiveness of moire effects is reduced in known manner, by the fact that the screen grids of the individual color separations of a color set are printed in superimposition in angularly staggered position with respect to each other. By virtue of the screen angle, the moire phases formed are in effect either too small or too large to be noticed as troublesome by the human eye. Color separations wherein the individual screen grids are turned through different screen angles with respect to the recording direction, are required for a screen rotation of this kind.
Consequently, four different screen angles are needed for the four color separations. To produce a moire minimum, it proved to be advantageous in four-color printing to select the screen angle -15.degree. for "magenta", the screen angle +15.degree. for "cyan", the screen angle 0.degree. for "yellow" and the screen angle +45.degree. for "black". The screen angles should be adhered to very precisely since troublesome moire effects already intervene at small angular deflections.
Other screen angles are then required in complementary fashion if other colors are to be printed, other print mediums are to be applied or if different screen line spacings are to be printed one over another.
The direct application of a screen on originals in the color scanner may for example be performed by means of a so-called contact screen application, wherein the recording beam is modulated in complementary fashion by the density variation of a contact screen film positioned between the recording element and the recording facility, to generate the screen grid elements.
For example U.S. Pat. No. 3,688,033 disclosed a method for so-called "electronic screen application" wherein each screen grid element is built up in the manner of a picture pattern from individual picture elements or type or body lines. The picture patterns of the different screen grid element sizes are stored as recording data for all tonal values and for different screen angles. The recording data are currently read out and recorded in each case, which correspond to the tonal values determined during scanning of the original, during the reproducing operation.
Whereas the instrument-related composition screen grid in which the screen grid elements are recorded is aligned orthogonally in the recording direction and feed direction of the implement, printing screen grids rotated in various angles with respect to the composition screen grid are decisive for the precise positional location of the screen grid elements on the recording medium.
What is required is to fit the different printing screen grids into the system of the printing lines. This is particularly uncomplicated according to U.S. Pat. No. 3,657,472 if the tangent of the screen angle is a simple rational number. A common areal element which has the fundamental structure of the screen pattern and which is repeated periodically on the recording medium in the recording and feed direction, whereby the recording operation is controllable by means of uncomplicated cadencing systems which are coupled to the displacement of the recording medium or with the feed motion of the recording element, then results for both screen systems in the case of such "rational screens".
Screen grids having screen angles whereof the tangent is irrational, cannot be recorded according to the method hereinabove described, so that the screen angles of plus and minus 15.degree. required for a moire minimum cannot be established either.
A different method, whereby "irrational screens" may also be recorded, is described in the U.S. Pat. No. 3,997,911. In this known method, XY pulse series are derived from the displacement of the recording drum and from the feed motion of the recording element, the analysis of said series determining the momentary positional locus of the recording element with respect to the recording facility in an orthogonal co-ordinate system aligned in the recording and feed direction.
The XY pulse series are converted in accordance with a predetermined function, to generate a screen signal. This function, which is periodic and bi-dimensional, represents the screen pattern turned through the required screen angle.
During the recording action, the screen signal and the image signal are compared continuously and the decision as to whether a screen grid element is to be or is not to be recorded at the locus characterised by XY pulse series is derived from the comparison.
The function is reproduced electrically in a function generator wherein, among others, other pulse series are initially generated by multiplication of the frequencies of the XY pulse series by particular factors, the factors being irrational or almost irrational and representing different functions of the screen angle selected for the printing operation.
The multiplication is performed by means of phase-locked loop circuits which, according to experience, have a build-up action and relatively low stability. The required screen angle may consequently be adhered to with a limited precision only, so that as already stated, troublesome moire phenomena may appear at a particular angular deviation.
To improve the definition and printability of the screen dots, it is frequently desirable to produce different screen dot shapes or to split the screen dot into partial elements, in accordance with U.S. Pat. No. 3,997,911 hereinabove referred to.
In the method disclosed by the U.S. Pat. No. 3,997,911, it may well be possible to produce circular or rectangular screen dots by means of different functions, but the possibilities of variation are very limited. Furthermore, some of the functions specified may be reproduced in a function generator with difficulty only, which is considered to be disadvantageous.
In the known device, the recording is produced by several partial beams situated one beside another, which are emitted from a recording element. The image signal must be compared to different screen signals, to control the partial beams. The generation of the screen signals which must make allowance for the different points of impingement of the partial beams on the recording facility, is not described in particular.